Preheat your oven to 350 degrees Fahrenheit. Spray three 8-inch round cake pans with cooking spray and dust with flour to prevent sticking.
In a medium bowl, sift together the flour, baking powder, baking soda, and salt. This ensures even distribution of the leavening agents and helps create a light cake.
In the bowl of your mixer, beat the butter on medium speed until it becomes creamy. This usually takes about 3 to 5 minutes. Add in the granulated sugar and continue to beat until the mixture turns light and fluffy, a process that should take an additional 3 to 5 minutes.
Now, it's time to incorporate the egg whites. Add them in one at a time, mixing on medium speed until each egg white is fully incorporated before adding the next. This step is crucial to achieving that fluffy texture.
Add the vanilla extract, milk, sour cream, and white vinegar to the butter mixture. Mix on medium speed until everything is well combined, creating a beautiful creamy batter.
Gradually add the dry ingredients from the bowl into the wet mixture, mixing until just combined. Be careful not to overmix, as this can lead to a denser cake.
Divide the batter evenly among the three prepared cake pans. Use a spatula to smooth the tops, ensuring they are level for even baking. Bake in the preheated oven for about 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
While the cakes are cooling, prepare the frosting. In the bowl of a stand mixer, combine the Mascarpone cheese and cream cheese. Using the paddle attachment, mix until the cheeses are well combined. Slowly add the heavy cream while mixing on low speed to prevent lumps.
Once the cream is fully incorporated, add the powdered sugar and remaining vanilla extract. Beat on medium speed until the frosting is smooth and creamy. If it appears runny, switch to the whisk attachment and whip it for a few minutes until it thickens.
Now, it’s time to assemble your cake! On a cake stand or plate, place one layer of cooled cake. Spread an even layer of the Chantilly cream over the top, followed by a layer of mixed berries.
Add the second layer of cake and repeat the frosting and berry layering. Finally, place the third layer of cake on top. Frost the sides and the top of the cake with the remaining cream. For an extra touch, you can pipe any remaining cream around the edges and garnish with additional berries.
Chill the assembled cake in the refrigerator until you’re ready to serve. This helps set the cream and makes slicing easier!